Implantable medical devices (IMDs) are implanted in patients to monitor, among other things, electrical cardiac activity, and to deliver appropriate cardiac electrical therapy, as required. IMDs include pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (ICD), and the like. The electrical therapy produced by an IMD may include pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm. IMDs can also be used to perform cardiac resynchronization therapy (CRT).
When IMDs are exposed to external magnetic fields, such as those produced by magnetic resonance imaging (MRI) systems, the magnetic fields may interfere with operation of the IMDs. For example, such external magnetic fields may generate magnetic forces on an IMD and the leads and electrodes attached to the IMD. These forces may induce electric charges or potentials on the leads and electrodes, which can cause over- or under-sensing of cardiac signals. For example, the charges may cause the electrodes and leads to convey signals to an IMD that are not cardiac signals, but are treated by the IMO as cardiac signals. This may cause the IMD to falsely detect tachycardias (which do not actually exist), potentially causing the IMD to delivery anti-tachycardia pacing (ATP) or defibrillation shock therapy (when not actually necessary). In another example, the charges induced by MRI systems may induce sufficient noise in cardiac signals such that cardiac signals that are representative of cardiac events go undetected by an IMD. This may cause the IMD to not detect a tachycardia (which actually exists), potentially causing the ND to not delivery appropriate anti-tachycardia pacing (ATP) or defibrillation shock therapy (when actually necessary). This may also cause the IMD to not deliver pacing therapy since it falsely believes there are intrinsic cardiac events ongoing.
An MRI system generally produces and utilizes three types of electromagnetic fields, which include a strong static magnetic field, a time-varying gradient magnetic field, and a radio frequency (RF) magnetic field, which can collectively be referred to as the magnetic field from an MRI system. The time-varying gradient field and the RF field may be referred to as different parts of the time-varying magnetic field. In other words, the time-varying gradient field and the RF field can collectively be referred to as the time-varying magnetic field. The static field produced by most MRI systems has a magnetic induction ranging from about 0.35 Tesla (T) to about 4 T, but can be potentially higher (e.g., 7 T and 9 T MRI systems are sometimes used in research). More specifically, MRI systems may generate external static magnetic fields having different strengths, such as 0.35 T, 0.5 T, 0.7 T, 1.0 T, 1.2 T, 1.5 T, 3 T, 4 T etc. The RF field includes RF pulses. The frequency of the RF field is related to the magnitude of the static magnetic field, with the frequency of the RF field being approximately 42.58e6* static field strength. For example, where the static magnetic field strength is 1.5 T, the RF is at 42.58e6*1.5˜64 MHz; and where the static magnetic field is 3 T, the RF is at 42.58e6*3˜128 MHz. The time-varying gradient magnetic field, which is used for spatial encoding, typically has a frequency in the KHz range, but for many MRI sequences can have relatively high power in the sub-KHz range.
In order to safely operate while exposed to magnetic fields produced by MRI systems, IMDs may switch modes to an “MRI safe mode”. Some IMDs require that a clinician send a telemetry command to the IMDs, via a special external programmer, in order to put the IMDs in an MRI safe mode. However, the need for this special external programmer and for clinician training on using the external programmer are time consuming, costly and cumbersome. Further, this protocol may not be properly followed, e.g., in emergency situations, when the technician operating the MRI system is not aware that the patient has an IMD, and/or when an appropriate external programmer is unavailable.
An IMD's failure to switch from its normal operational mode into an MRI safe mode, when it should have, may cause the IMD to inhibit necessary pacing, or delivery unnecessarily high voltage therapy or anti-tachycardia pacing, which may induce an arrhythmia. Further, failure of an IMD to switch out of an MRI safe mode and back to its normal operational mode, when it should have, may cause pacing that leads to non-optimal therapy, loss of rate-response, pacemaker syndrome, and/or other problems.
In order to sense and detect external magnetic fields, some IMDs include giant magnetoresistance (GMR) sensors. Known GMR sensors are typically configured to detect magnetic fields of relatively small magnitudes produced by a handheld magnet. The GMR sensor operates by detecting a change in an electrical resistance characteristic of the sensor when the sensor transitions from not being exposed to a magnetic field to being exposed to a magnetic field. In response, the IMD may switch to a “magnet mode” of operation. During the magnet mode of operation, the IMD may, e.g., pace the ventricle(s) at a predetermined fixed rate without sensing cardiac signals or responding to any detected cardiac events. Alternatively, or additionally, when in the magnet mode the IMD may record of an intracardiac electrogram (IEGM) for subsequent evaluation. The IMD's operation when in “magnet mode” may depend on the brand of IMD, the type of IMD, the level of battery charge in the device, and more generally, how the magnet mode is defined for the specific IMD and/or patient. In some IMDs, the magnet mode may shut off the device. As the terms are used herein, a magnet mode and an MRI safe mode refer to different modes of operation for an IMD, although there may be some overlap as to how the IMD operates in its magnet mode and its MRI safe mode (e.g., in both modes, the IMD may pace without sensing cardiac signals). It is even possible that a physician may program the magnet mode and the MRI safe mode to be the same or similar for a specific patient.
Conventional GMR sensors used in IMDs are typically formed from materials that may become saturated when exposed to relatively small magnetic fields, and most likely will become saturated when exposed to the relatively strong magnetic fields produced by MRI systems. For example, some known GMR sensors become saturated when exposed to magnetic fields of as low as 15 Gauss (G), where 1 G=1×10^−4 T. Once the GMR sensor is saturated, further increases in the external magnetic field are not detected by the GMR sensor. Accordingly, conventional GMR sensors may be unable to reliably sense relatively strong external magnetic fields. As a result, the GMR sensors may be incapable of detecting the presence of external magnetic fields generated by MRI systems. Also, GMR sensors may be unable to differentiate between different strengths of magnetic fields. For example, GMR sensors may be incapable of differentiating between relatively small external magnetic fields (e.g., produced by a relatively small handheld magnet) intended to switch an IMD into its magnetic mode and/or in which the IMD may continue to safely operate, and relatively strong external magnetic fields generated by an MRI system, in which the IMD may be unable to safely operate unless switched into an MRI safe mode. For another example, where a patient's legs (or head) are within the high static magnetic field of an MRI system, while the patient's torso (in which an IMD with a GMR sensor is implanted) is outside the high static magnetic field of the MRI system, the magnitude of the magnetic field detected by the GMR sensor may be similar to that of a handheld magnet. This may cause the IMD to switch into its magnet mode, when it actually should have switched into an MRI safe mode. Similar problems to those discussed above with regard to GMR sensors can also arise where an IMD includes a Hall effect sensor or a reed switch for the purpose of detecting a handheld magnet. For example, where an IMD includes a Hall effect sensor for the purpose of detecting a handheld magnet (for use in switching the IMD to a magnet mode), the Hall effect sensor may not be able to distinguish between magnetic fields produced by a handheld magnet and an MRI system (e.g., where a patient's legs or head are within the high static magnetic field of an MRI system, while the patient's torso is outside the high static magnetic field of the MRI system).
Therefore, a need still exists for IMDs, and methods for use therewith, that can accurately detect the exposure of the IMDs to magnetic fields generated by MRI systems.